Variable Volume, Shape Memory Actuated Insulin Dispensing Pump

ABSTRACT

A portable pumping system provides insulin or other drugs to a user. A shape memory element is used to actuate the pump and an intelligent system controls the actuator in order to minimize stresses within the system and provide accurate and reliable dosage delivery. The control system utilizes various types of feedback to monitor and optimize the position of the pumping mechanisms. Physical design aspects also minimize stress and the combination of the physical design aspects and the intelligent operation of the system results in a lightweight and cost effective pump that may be used in a disposable fashion if desired.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. applicationSer. No. 10/683,659 of Benjamin M. Rush et al., filed on Oct. 9, 2003,which is related to and claims priority based on U.S. ProvisionalApplication No. 60/417,464, entitled “Disposable Pump for Drug DeliverySystem,” filed on Oct. 9, 2002, and U.S. Provisional Application No.60/424,613, entitled “Disposable Pump and Actuation Circuit for DrugDelivery System,” filed on Nov. 6, 2002, each of which is herebyincorporated by this reference in its entirety. The parent application,U.S. application Ser. No. 10/683,659, was published as U.S. PatentApplication Publication No. 2004/0115067 A1. The present application isrelated to U.S. application Ser. No. ______, of Benjamin M. Rush,entitled “Fluid Delivery Device with Auto Calibration,” and U.S.application Ser. No. ______ of Benjamin M. Rush, entitled “Devices AndMethods For Use in Assessing a Flow Condition of a Fluid,” each of whichis filed concurrently with the present application and is herebyincorporated herein, in its entirety, by this reference.

FIELD OF THE INVENTION

The present invention is generally related to portable insulin or otherliquid delivery systems and more specifically related to a pump for usein such systems.

BACKGROUND OF THE INVENTION

Insulin pumps are widely available and are used by diabetic people toautomatically deliver insulin over extended periods of time. Manycurrently available insulin pumps employ a common pumping technology,the syringe pump. In a syringe pump, the plunger of the syringe isadvanced by a lead screw that is turned by a precision stepper motor. Asthe plunger advances, fluid is forced out of the syringe, through acatheter to the patient. The choice of the syringe pump as a pumpingtechnology for insulin pumps is motivated by its ability to preciselydeliver the relatively small volume of insulin required by a typicaldiabetic (about 0.1 to about 1.0 cm3 per day) in a nearly continuousmanner. The delivery rate of a syringe pump can also be readily adjustedthrough a large range to accommodate changing insulin requirements of anindividual (e.g., basal rates and bolus doses) by adjusting the steppingrate of the motor. While the syringe pump is unparalleled in its abilityto precisely deliver a liquid over a wide range of flow rates and in anearly continuous manner, such performance comes at a cost. Currentlyavailable insulin pumps are complicated and expensive pieces ofequipment costing thousands of dollars. This high cost is due primarilyto the complexity of the stepper motor and lead screw mechanism. Thesecomponents also contribute significantly to the overall size and weightof the insulin pump. Additionally, because of their cost, currentlyavailable insulin pumps have an intended period of use of up to twoyears, which necessitates routine maintenance of the device such asrecharging the power supply and refilling with insulin. These syringetype pumps, even if described as disposable, are simply too expensive tobe truly disposable, or are alternatively disposed at a very high costto patients and insurance companies alike.

Shape memory alloys are a part of a class of materials that change shapewhen power is applied to them but that return to their natural statewhen the power is removed. The materials can be used to form an actuatorby harnessing this unique attribute of the materials. A pump can be madewith a shape memory alloy actuator. However, a shape memory alloy doesnot have the inherent accuracy and repeatability of the precisionstepper motor used in a syringe pump. Although price is alwaysimportant, precision is also essential in a pump used to deliver insulinor other drugs. It is therefore necessary to provide a system toprecisely control and actuate a pump utilizing a shape memory materialas an actuator.

SUMMARY OF INVENTION

The present invention employs a cost effective yet precise pumpingsystem and method to deliver insulin or other liquid to a user. Uniquephysical design aspects and an intelligent control system employed inthe present invention allow for a shape memory alloy to actuate apumping mechanism with excellent reliability and repeatability.

The present invention allows for not only a cost effective pumpingsystem, but also for a robust, precise, light weight, and fault tolerantsystem. Although the pumping system is precise, light weight, and faulttolerant, in the medical applications where the pump will be mostadvantageous, numerous reasons may make it desirable to dispose of andreplace portions of the pumping system relatively frequently. The lowcost of the pumping mechanism of the present invention allows for suchdisposable usage, while at the same time the pump is able to provideprecision doses throughout the life of the pump. Stresses in the pumpare minimized with the control system, and warnings can be generated ifthe pump is not primed properly or if an occlusion is detected withinthe pumping system. The reduction of stresses within the pump providesfor a smaller and lighter weight pump with a longer lifetime, which isof obvious benefit to a user of the pump. Furthermore, the intelligentcontrol system allows the pump to operate even if a fault is detected.For example, if the full stroke of the pump is unavailable for somereason, a lesser stroke can be utilized (at a higher frequency) and thepump can continue to provide the necessary dosage to the user.

Additional aspects, advantages and features of the present invention areincluded in the following description of exemplary examples thereof,which description should be taken in conjunction with the accompanyingfigures, and wherein like (and similar) numerals are used to describethe same feature throughout the figures. While the prefix of a numberingelement may change based upon the figure number, if the remainder of thenumbering element is the same in the various embodiments, the componentis the same or similar to that described regarding an earlier describedembodiment. For example, capacitor 304 of FIG. 3 is the same or similarto capacitor 504 of FIG. 5. When this is the case, the element will notbe described again, and reference should be made to the description ofthe earlier figure (FIG. 3 in this example). All patents, patentapplications, articles and other publications referenced herein arehereby incorporated herein by this reference in their entirety for allpurposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C illustrate pump 100 at various stages of operation.

FIGS. 1D is a block diagram of pumping system or “pump” 150.

FIGS. 2A, 2B, and 2C illustrate pump 200 at various stages of operation.

FIGS. 3A and 3B illustrate different embodiments of pump drive circuitsfor use with pump 200 or other pump embodiments.

FIGS. 4A and 4B illustrate pump 400 at various stages of operation.

FIG. 5 illustrates an embodiment of a pump drive circuit for use withpump 400 or other pump embodiments.

FIGS. 6A and 6B illustrate pump 600 at various stages of operation.

FIGS. 7A and 7B illustrate pump 700 at various stages of operation.

FIG. 8 illustrates an embodiment of a pump drive circuit for use withpump 700 or other pump embodiments.

FIGS. 9A and 9B illustrate pump 900 at various stages of operation.

FIGS. 9C, 9D, and 9E illustrate different embodiments of positionencoding utilized for linear feedback.

FIG. 10 illustrates an embodiment of a pump drive circuit for use withpump 900 or other pump embodiments.

FIG. 11A is a graph of a pump operating in an unprimed state.

FIG. 11B is a graph of a pump operating in a primed state.

FIG. 11C is a graph of occlusion detection within a pump.

FIGS. 12A and 12B are graphs of pump operation over time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention employs a cost effective yet precise pumpingsystem and method to deliver insulin or other liquid to a user. Uniquephysical design aspects and an intelligent control system employed inthe present invention allow for a shape memory alloy to actuate apumping mechanism with excellent reliability and repeatability.

The present invention allows for not only a cost effective pumpingsystem, but also for a robust, precise, light weight, and fault tolerantsystem. Although the pumping system is precise, light weight, and faulttolerant, in the medical applications where the pump will be mostadvantageous, numerous reasons may make it desirable to dispose of andreplace portions of the pumping system relatively frequently. The lowcost of the pumping mechanism of the present invention allows for suchdisposable usage, while at the same time the pump is able to provideprecision doses throughout the life of the pump. Stresses in the pumpare minimized with the control system, and warnings can be generated ifthe pump is not primed properly or if an occlusion is detected withinthe pumping system. The reduction of stresses within the pump providesfor a smaller and lighter weight pump with a longer lifetime, which isof obvious benefit to a user of the pump. Furthermore, the intelligentcontrol system allows the pump to operate even if a fault is detected.For example, if the full stroke of the pump is unavailable for somereason, a lesser stroke can be utilized (at a higher frequency) and thepump can continue to provide the necessary dosage to the user.

As mentioned briefly above, a shape memory alloy is used to actuate apump made in accordance with the present invention. In the process ofundergoing a dimensional change, the shape memory material goes througha reversible phase transition or transformation, or a reversiblestructural phase transition, upon a change in temperature. Generally,such a transition represents a change in the material from one solidphase of the material to another, for example, by virtue of a change inthe crystal structure of the material or by virtue of a reordering ofthe material at a molecular level. In the case of nitinol, for example,the superelastic alloy has a low temperature phase, or martensiticphase, and a high temperature phase, or austenitic phase. These phasescan also be referred to in terms of a stiff phase and a soft andmalleable phase, or responsive phase. The particular phase transitionassociated with a particular alloy material may vary. Shape memorymaterials are well understood by those of ordinary skill in the art.

Pump 100, an embodiment of a pump (or a portion thereof) of the presentinvention, is shown in the inactive state in FIG. 1A, the fullyactivated state in FIG. 1B, and the stress-loaded state in FIG. 1C. Thepump body comprises a case 101, a top cap 102, and a plunger cap 103.Within the pump is a plunger 104 that is normally (in the inactivestate) held against the plunger cap 103 by a plunger bias spring 105.Similarly, an overload piston 106 is normally (in inactive state) heldagainst the top cap 102 by an overload piston spring 107 which isstronger (has a higher spring constant k) than the plunger bias spring105. The plunger 104 is connected to the overload piston 106 by a shapememory alloy wire 108 which contracts when heated by a pulse or pulsesof current flowing from the V+ 109 contact to the V− 110 contact throughthe shape memory alloy wire 108 where the V− 110 contact may be thesystem ground (GND) reference. The power in each pulse is determined bythe voltage applied to the shape memory alloy wire 108 through the V+109 and V− 110 contacts. It is worth noting that the case is made of aninsulating material while the plunger 104 and overload piston 106 areeither made of a conductive material (e.g. metal) or are coated with anappropriately conductive material. The top cap 102 and plunger cap 103may be made of insulating or conductive material as is best suited to agiven design.

FIG. 1A shows the pump in the inactive state where the shape memoryalloy wire 108 is not contracted, the plunger 104 is held against theplunger cap 103 by the plunger bias spring 105 and the overload piston106 is held against the top cap 102 by the overload piston spring 107.This is the state to which the pump 100 returns after each activation orpumping cycle.

FIG. 1B shows the pump in the fully activated state where the shapememory alloy wire 108 has contracted enough to pull the plunger 104 upagainst a stop built into the case 101 without moving, while overloadpiston 106 which is held against the top cap 102 by the overload pistonspring 107. This state realizes a full stroke of the plunger 104.

FIG. 1C shows the pump in the stress-loaded state where the shape memoryalloy wire 108 has contracted sufficiently to pull the overload piston106 up against a second stop built into the case 101. In this state thecase 101, plunger 104, overload piston 106, and shape memory alloy wire108 are under maximum stress.

The design of the basic pump 100 is such that there is no feedback tothe circuit driving the pump (open loop) and the action of the pumpafter the fully activated state shown in FIG. 1B is accommodated by thedesign margin to ensure that the pump reaches a fully activated state.If the pulse or pulses of current applied to the shape memory alloy wire108 are reduced to the minimum value required to achieve the fullyactivated state under worst case conditions, such as a cold wire, thenthe action of the basic pump 100 under best case conditions, such as awarm wire, will drive the pump toward the stress-loaded state shown inFIG. 1C. The design of the pump 100, and the selection of the overloadpiston spring 107 is driven by the differences between the worst-caseand best-case conditions. Under normal operating (non-fault) conditionsthe pump always completes the full stroke (the fully activated state) asshown in FIG. 1B and operates reliably over the expected life of thepump because excess contraction and the resultant stress are minimized(as seen in the stress-loaded state shown in FIG. 1C). Considerationsfor the worst-case and best-case conditions include operatingtemperature range, the minimum pumping rate (e.g. the minimum basaldelivery rate), and the maximum pumping rate (e.g. the maximum bolusrate).

It is important to note that the open-loop design of pump 100 lacksfeedback and thus cannot adaptively accommodate faults as they are notsensed. For example, a pump failure such as a jammed plunger 104 couldcause a reduced or zero insulin delivery output and the pump would beassumed by the user (patient) to be operating correctly when an improperdose was delivered.

FIG. 1D is a block diagram that shows the overall system of which thevarious pump embodiments are a part. The overall system 150 comprises amicroprocessor 150A, drive circuitry 150B, and pump element 150C. All ofthese components can be considered to form the pump, even though pumpelement 150C alone is also sometimes referred to as the pump among thoseskilled in the art. Many different embodiments of the pump 150C and of aportion of the drive circuitry 150B are described in detail below, andthroughout the application. In an insulin delivery system 150, all ofthe components (that are shown) may be packaged together oralternatively they may be grouped separately. For example, it may bedesirable to group the pump and drive circuitry together while remotelylocating the pump element. Other components such as user input devicesand a display are not shown, but are all controlled by the processor inconjunction with the pump and drive circuitry.

Another embodiment is illustrated in FIG. 2. The design shown in FIG. 2comprises feedback that indicates the completion of the fully activatedstate but is otherwise similar to the pump shown in FIG. 1. The pump 200incorporates feedback from a switch (“PISTON_NC 211”) that indicatesthat the overload piston 206 is at the top of the pump or in contactwith top cap 202. A switch, such as switch 211 (that provides feedback)may alternatively be referenced for the feedback it provides in thefollowing description. The pump with PISTON_NC 211 feedback shown inFIG. 2 is constructed and operates in a similar fashion to the basicpump 100 shown in FIG. 1. The feedback comes from a normally-closed (NC)switch that indicates the overload piston 206 is in contact with the topcap 202 as in FIG. 2A and FIG. 2B. When the pump 200 enters the overloadstate as shown in FIG. 2C then the switch opens and feedback is fed tothe drive circuit. If the feedback is not received during the maximumpulse period used for pump 100 then an error has occurred and pump 200operation can be discontinued. The PISTON_NC 211 feedback is shown asconnected directly to the top cap 202 which indicates that the top cap202 is either made of a conductive material (e.g. metal) or is coatedwith an appropriately conductive material. If the design of a given pumprequires the top cap 202 to be made of an insulating material then thePISTON_NC 211 feedback can be moved to the inner surface of the top cap202 so that the PISTON_NC 211 feedback is in direct contact with theoverload piston 206 in the inactive state as shown in FIG. 2A.

An advantage of pump 200 is fault detection based on the feedback from(normally closed) switch 211 (if the switch is not activated in themaximum pulse duration). The pump also saves energy because itterminates the activation pulse when full pump action is achieved.Minimizing energy consumption is extremely important for a portableinsulin pump, as it maximizes the time the pump can be used withoutinconveniencing the user.

FIG. 2C shows the pump in the stress-loaded state where the shape memoryalloy wire 108 has contracted sufficiently to pull the overload piston206 down, but not up against a stop built into the case 201. In thisstate, the case 201, plunger 204, overload piston 206 and shape memoryalloy wire 208, are under stress. However, that stress is limited to thespring constant (k) of the overload piston spring 207 and is thusreduced as compared to the stress-loaded state shown in FIG. 1C wherethe overload piston 106 is against a hard stop of the case 101. Themethod used to further reduce the already minimized stress is thetermination of the pulse or pulses of current that are flowing from theV+ 209 contact to the V− 210 contact through the shape memory alloy wire208. This causes the shape memory alloy wire 208 to stop contracting andthus reduces the stress on the pump 200.

There are two primary methods to terminate the pulse or pulses to theshape memory alloy wire 208 as shown in FIG. 3A and FIG. 3B. The actualdrive circuits are identical and the only difference between FIG. 3A andFIG. 3B is in the Voltage Output (Vout) and feedback connections asdiscussed below. Each drive circuit is connected to a power source VCC301 and to the system ground GND 302. Each has a pull-up resistor R 303from the feedback to VCC 301 and an optional filtering or “debounce”capacitor C 304 from the feedback to GND 302. The feedback is digitaland detects a logic ‘0’ when approximately 0V or GND 302 is present(i.e. the switch is closed) and a logic ‘1’ when a voltage approximatelyequal to the supply voltage or VCC 301 is present (i.e. through thefunction of the pull-up resistor R 303 when the switch is opened). Ifthe optional filtering or “debounce” capacitor C 304 is not present thenthe feedback may oscillate briefly when the switch opens or closes dueto mechanical vibration related to the switch contact. If the optionalfiltering or “debounce” capacitor C 304 is present then the feedbackactually detects the voltage on the capacitor C 304 which can not changeinstantaneously. When the switch closes the capacitor C 304 will bedischarged quickly to approximately 0V or GND 302; when the switch opensthe capacitor will be charged at a rate proportional to the values ofthe resistor R 303 and the capacitor C 304 to approximately the supplyvoltage or VCC 301. For example, a resistor R 303 value of 10,000 Ohms(10 kΩ) and a capacitor C 304 value of 100 pF would have a time-constantof one microsecond (1 μsec) and the state of the feedback would changefrom a logic ‘0’ to a logic ‘1’ in about two microseconds (2 μsec)without any oscillations (noise) on the feedback that could be actedupon by the drive circuit inappropriately.

The first method as shown in FIG. 3A is to connect the PISTON_NC 211 tothe feedback to gate the drive signal Vout that is created by the drivecircuit and which is connected to the pump V+ 209 contact. When thedrive circuit receives feedback that the overload state is entered asshown in FIG. 2C then the pulse or pulses can be terminated and both thestress is reduced and power is saved. The second method as shown in FIG.3B is to provide power to the pump 200 through the PISTON_NC 211 contactrather than the V+ 209 contact. This method automatically removes powerfrom the shape memory alloy wire 208 whenever the PISTON_NC 211 switchopens as shown in FIG. 2C. If the feedback is ignored (i.e. the drivecircuit is simplified to remove the feedback), then the overload piston209 may oscillate between the states shown in FIG. 2B and FIG. 2C untilthe pulse or pulses from the drive circuit are terminated and only apartial power saving is realized. If the feedback is utilized as in FIG.3A then when the drive circuit receives feedback that the overload stateis entered as shown in FIG. 2C, the pulse or pulses can be terminated toprevent oscillations, and maximum power saving is realized as in thefirst method.

Addition of the PISTON_NC 211 feedback reduces the overall forcesgenerated within the pump and allows the pump to be made smaller andlighter with improved reliability. Unfortunately, if the plunger 204jams then the overload piston will begin moving and provide feedbackthat indicates the pump is operating properly. Again, a jammed plunger204 could cause a reduced or zero insulin delivery output, but in thissituation the pump would be assumed by the user (patient) to beoperating correctly when in fact an improper dose may have beendelivered.

Another embodiment of the invention is seen in pump 400 of FIGS. 4A and4B. Pump 400 incorporates feedback that (more directly) indicates thecompletion of the fully activated state. Pump 400 uses (PLUNGER_NO)switch 411 to indicate that the plunger 404 is against the upper stop.This switch is used in place of (or in conjunction with) switch 211, andall of the feedback control and stress limitation features describedwith respect to pump 200 are present in pump 400. Drive circuit 500 seenin FIG. 5 is similar to drive circuit 300, as previously described. Pump400 can also detect a fault with the pump if the plunger is not where itis expected to be based upon the potential applied to the actuator, aswas also described previously. Similarly, the pump can detect a jam ifthe plunger is not where it is expected to be based upon the potentialapplied to the actuator.

Another embodiment of the invention is seen in pump 600 of FIGS. 6A and6B. Pump 600 is functionally the same as pump 400 but lacks overloadpiston 406 and overload spring 407. Because of the lack of these items,the top cap 607 preferably has some amount of compliance and acts as asimplified spring. Pump 600 has fewer parts and is thus lighter andsmaller than pump 600. Fewer parts also generally result in improvedreliability over the life of the pump.

Yet another embodiment of the invention is seen in pump 700 of FIGS. 7Aand 7B. Pump 700 is similar to pump 600 with the added advantage offeedback switch 710 (PLUNGER_NC) that directly indicates the completionof the fully activated state and return to the inactive state (at thecompletion of a pump cycle). Because pump 700 “knows” when a pump cycleis completed (and when it should be completed) it therefore “knows” whenthere is a fault, and can accommodate for the fault in what is known asa fault tolerant design. The fault tolerance is in both the directmeasurement of the plunger 704 action and in ensuring that the plungeris resting in the fail safe state after the maximum permissible pumpcycle time (this may also indicate a major occlusion in the pumpsystem). If the power (GND) to the V− 708 contact is switched (via aseries switch) to provide additional fault tolerance as is done in somepump systems, then the added feedback will also indicate the state ofthe V− 708 switch (not shown for clarity sake) as the value of switch710 (PLUNGER_NC) will be 0V (GND) when the series power switch is closedand VCC when the series power switch is open. The pump can also detectan occlusion if the plunger does not return to the fully down state inthe maximum pump cycle time.

The PLUNGER_NC 710 feedback is shown as connected directly to theplunger cap 703 which indicates that the plunger cap 703 is either madeof a conductive material (e.g. metal) or is coated with an appropriatelyconductive material similar to the top cap 202 of FIG. 2. If the designof a given pump requires the plunger cap 703 to be made of an insulatingmaterial then the PLUNGER NC 710 feedback can be moved to the innersurface of the plunger cap 703 so that the PLUNGER NC 710 feedback is indirect contact with the plunger 704 in the inactive state as shown inFIG. 7A. Drive circuit 800 illustrated in FIG. 8 is similar to the drivecircuits previously described. The pump 700 and drive circuit 800comprise the minimum configuration for a fault tolerant system. All ofthe linear feedback techniques described below add fault resolution andimprove fault tolerance at the expense of added cost and complexity.

Linear Feedback

Embodiments of a pump as previously described may also comprise linearfeedback that directly indicates the position of the plunger. The linearfeedback may be analog or digital and is used to detect the position ofthe plunger. The linear feedback may also indicate if there is a faultbased upon the position of the plunger during various phases ofoperation of the pump. The linear feedback system can employ conductiveencoding marks. This is a simple and economical way to detect theposition of the plunger. Alternatively, optical position sensingutilizing optical encoding marks may be employed. This is more precisebut is also more complex and expensive.

FIGS. 9A and 9B illustrate pump 900, another embodiment of the presentinvention. Pump 900 is similar to pump 700 but employs direct linearfeedback in addition to the feedback provided by the switches. Thisfeedback is contained in a linear feedback signal (“LINEAR_FB”) 911shown in the figures. Linear feedback may also be used to detect primingof the pump, which will be described later with regard to FIGS. 11.

FIG. 9C illustrates one possible embodiment of position encoding, oneway of providing linear feedback. In this embodiment the encoding schemeutilizes conductive encoding marks. One way to create the encoding gridis with insulating paint silk-screened onto a conductive surface so asto insulate certain areas. This conductive coating would be on the sideof the moving part. For example, it could be directly on the piston oron an attachment to the piston. The black areas of the grid are themetal surface without paint on top of them. The white areas of the gridare covered with the insulating paint. The black row (long conductingstrip) at the top is a reference ground. When contacts 930 touch theblack squares then they are shorted to ground. When shorted to groundthey are said to form a “1” whereas when they are not they form a “0.”This logic can be inverted if desired.

In the position depicted in FIG. 9C, the ground contact is insulatedfrom the most significant bit (“MSB”) contact and the least significantbit (“LSB”) contact. Therefore it is at position 0 (binary position 00).As this moving part slides left under the contacts 930, then position 1(binary position 01) will next be sensed. When the part slides leftagain position 2 (binary position 10) will next be sensed etc. FIG. 9Cillustrates 4 positions, that is 2 bits of encoding for illustrativepurposes. However, this can be extended to any number of positions. Forexample, 32 positions would require 5 bits. This digital positionsensing can be used for digital feedback and control of the piston, andthus can be used to control position of the piston and the amount ofinsulin delivered.

Optical encoding may be employed instead of the conductive encodingdescribed above. Instead of shorted contacts, an optical sensor (anLED+photocell, for example) is used to sense if the shiny metal ispresent or if light absorbing black paint is present.

A minor modification to the encoding shown in FIG. 9C is shown in FIG.9D. In FIG. 9D the encoding marks or bits are laid down in a Grey code.That is, only one bit change is allowed per position. Grey codes haveseveral desirable properties that are well known in the art.

Degradation of the contacts and various other parts can occur over time.For example, contacts can be dirty, worn, or broken, and contaminationmay cause faulty contact readings, etc. This would normally cause anerror or misread. There are various ways to minimize the errors and tocorrect any errors that may occur. In one method, additional bits areadded to the surface. A single bit (called a parity bit) can be added todetect some kinds of errors. Multiple bits can be added for even moreerror protection. With several added bits errors can be both detectedand corrected. A measure of this is the Hamming distance, which is wellknown in the art. Briefly stated, the Hamming distance can beinterpreted as the number of bits which need to be changed to turn onestring into the other. Sometimes the number of characters is usedinstead of the number of bits.

Error detection and correction theory is a well known science used as apart of radio communications theory, and can be applied to the encodingand position recognition mechanisms of the present invention. Thisincludes BCH codes, parity codes, and Reed-Solomon codes, etc. Thesystem of FIG. 9E includes a parity bit that can be used for errorcorrection encoded on the moving object.

Analog sensing of position can be made by plating two plastic, insulatedsurfaces with metal, or alternatively simply providing two metalsurfaces. The two surfaces are used as capacitor plates—and togetherform a capacitor. One capacitor plate would be stationary, while theother capacitor plate would be part of the moving assembly including thepiston. The measured capacitance is proportional to the distance betweenthe plates, and therefore can be used to measure the position of thepiston. This analog position sensing can be used for feedback andcontrol of the moving part.

Analog sensing of position can also be achieved with magneticmeasurements by adding a magnet to the moving part and sensing on thestationary part. Similar to the capacitance measurement described above,the magnetic field will vary depending on the distance between themoving and stationary parts. Therefore, the magnetic sensor may be usedto measure the position of the piston and this type of analog positionsensing can be used for feedback and control of the moving part. Onetype of well known magnetic sensor is a Hall Effect sensor, but anymagnetic sensor may be utilized.

Resistance measurements may be used to implement analog linear feedback.Similar to a potentiometer, the piston will have different resistancevalues the further a measurement is taken along the length of theplunger. In other words, the resistance will increase with distance acurrent must travel.

Usage of the linear feedback has many advantages. One advantage ofemploying the feedback is that the drive circuit can “servo” the plungeror control the position or stroke of the plunger with a relatively highdegree of accuracy. Thus, a partial plunger stroke may be used to givefiner dose delivery, and that dose can be any fraction of the pumpcylinder volume. By measuring and controlling the plunger movementvariable size rather than only discrete volume doses may beadministered. Additionally, a partial plunger stroke may not only bedetected when it is undesirable (as in pump 700), but the volume of thepartial stroke may be measured and compared to the expected volume thusadding fault resolution. For instance, if a full stroke was supposed totake place and deliver a certain volume, the system can detect that lessthan the desired volume was pumped and make up for the missing volume orindicate a failure condition with a measure of the error being reported.A pump having position detection and control is more fault tolerant thana pump without it. For example, if a certain portion of the full strokerange is unavailable for some reason, the pump can control the stroke toonly use the available range. This could provide invaluable additionaloperating time in what would otherwise be a malfunctioning orinoperative pump. For a diabetic who must have insulin the value of thisis potentially life-saving.

Priming, Fault Tolerance, and Servo Control

Another improvement to the basic pump design is to monitor the feedbackas an indication of the operation of the entire pump system and not justthe proper functioning of the plunger. FIG. 11A shows a pump prior tobeing “primed” where there is air in the pump system leading to thepatient including the tubing and infusion set (the portion attached tothe user where insulin is delivered to the user's tissue).

Using pump 900 as an example, although application in other embodimentssuch as pump 700 is also possible, at time t=0 (the initial timereference) the pump 900 is activated (the V− 908 switch is enabled ifpresent and power is applied to the V+ 907 contact by the drive circuit1000) as is shown in FIG. 9A. At time t=1 the plunger 904 begins to moveand PLUNGER_NC 910 changes state from a Logic ‘0’ to a Logic ‘1’ toindicate plunger 904 movement. At time t=2 the plunger 904 activates thePLUNGER_NO 909 contact which changes state from a Logic ‘1’ to a Logic‘0’ to indicate the plunger 904 has achieved a full upward stroke as isshown in FIG. 9B. This causes power to be removed by the drive circuit1000 via the feedback (FB_NO) and shortly thereafter the plunger beginsto fall and the PLUNGER_NO 909 contact changes state from a Logic ‘0’ toback to a Logic ‘1’ as affirmed by the drive circuit 1000 feedback. Attime t=3 the plunger 904 has completed a full pump cycle and PLUNGER_NC910 changes state back from a Logic ‘1’ to a Logic ‘0’ to indicate thecompletion of a full pump cycle as shown again in FIG. 9A (at this timethe V− 908 series power switch is disabled if present to preventpossible pump “misfires” due to noise or other system errors). Thedigital feedback provides a simple and clear indication of a fault.

The same cycle is shown in FIG. 11B where the pump system is fullyprimed and operating as compared to the unprimed state shown in FIG.11A. The time from t=1 to t=2 is shorter in FIG. 11A than in FIG. 11B asthe pump 900, specifically the plunger 904, is pulling air from thereservoir versus insulin. This may be due to the initial priming whereair is being purged from the system or due to a reservoir failure.Similarly the time from t=2 to t=3 is shorter in FIG. 11A than in FIG.11B as the pump 900, specifically the plunger 904, is pushing airthrough the tubing and infusion set versus insulin. In fact, the timefrom t=2 to t=3 may be used to detect a fully primed pump that is readyfor insertion. If the tubing or infusion set were to break afterinsertion than the time from t=2 to t=3 would decrease and a fault couldbe detected. This phenomenon is similar to the affect of having air inbrake hydraulic lines on an automobile where the brake feels soft due tothe compressibility of air versus fluid. Priming the pump is analogousto “bleeding” the brakes. When the pump is primed it takes more energyto push the fluid through the tubing and infusion set. This pressure mayincrease even more when the insulin is pushed into the user's body(tissue). Since the plunger 904 is driven by the plunger spring 906, theextra force becomes related to time and is measured as the time from t=2to t=3.

In fact, the priming techniques described above may be used toautomatically prime a pump under the control of the microprocessor 150A.Rather than have the user prime the pump manually, and stop when fluid,such as insulin, begins to emerge from the tip of an infusion set (notshown), the pump can use the feedback described above to prime the pumpautomatically and optionally ask the user to confirm that priming iscomplete. The priming can include the entire infusion set or otherattachment to the pump, and not just the pump itself. This enhancementis especially important for young pump users and those who are visionimpaired or otherwise have poor eyesight. Those users can rely on theautomatic priming and can (optionally) confirm the priming by feelingthe liquid as it exits the final point to be primed.

This automatic priming technique also applies in a similar fashion toother pump systems. For example, on a syringe pump system with a steppermotor, the power to the motor when monitored is an indication of thework done by the motor in a fashion analogous to work done by theplunger spring 906. The work would be monitored by a shunt resistor usedto measure the motor current, or alternatively the droop in the batteryor power supply would be monitored to indicate power used by the motorand thus work done by the pump.

FIG. 11C illustrates the occurrence and detection of an input occlusion(increase in time from t=1 to t=2) and output occlusion (increase intime from t=2 to t=3). This system preferably accounts for circuitvariation and battery voltage droops so that these conditions areerroneously interpreted as an input or output occlusion.

The actuation of the plunger or piston can be modified or servocontrolled to make the pump operate more efficiently and to reducestress on the pump. This would allow for a smaller and lighter pump withimproved reliability.

FIG. 12A is a graph illustrating pumping operation over time. The timesin FIG. 12A correspond to the times shown in FIG. 11B. The rate ofchange of the position, as indicated by linear feedback signal 911increases over time until the piston reaches the top of its travel attime t=2. This will result in significant stress when the piston hitsthe hard stop.

FIG. 12B is a graph illustrating pumping operation over time where thepiston movement is modulated to reduce the acceleration and velocity ofthe piston before it hits the hard stop. This will reduce the amount ofstress encountered by all of the moving parts of the pump. At time t=0.5the power from the drive circuit 1000 is reduced to reduce the stress(impact) at time t=2. This can include pulse width modulation (“PWM”) ofthe potential applied to the shape memory element. For example, the PWMrate may be modified to a new value or changed per a specified profile.Similar modification to the action of the piston could modify theprofile leading to t=3 by adding occasional small pulses of energy toslow the descent of the plunger 904.

Although the various aspects of the present invention have beendescribed with respect to exemplary embodiments thereof, it will beunderstood that the present invention is entitled to protection withinthe full scope of the appended claims.

1. A pumping system comprising: a chamber; a piston within the chamber;a shape memory element within the chamber that changes length when acurrent flows through the element, the element moving the piston betweena first and second physical limit, such that when the piston is at thefirst limit the chamber is occupied with liquid and when the chamber isat the second limit the chamber is substantially discharged of liquid;and a first switch at the first limit that indicates if the piston is atthe first limit.
 2. The pumping system of claim 1 further comprising asecond switch at the second limit that indicates if the piston is at thesecond limit.
 3. The pumping system of claim 2 further comprisingcircuitry that senses the condition of the first and second switches. 4.The pumping system of claim 3 wherein the circuitry is operable todetect a pump malfunction if the piston is not at the first or secondlimit.
 5. A method of dispensing liquid, comprising: drawing liquid intoa pumping chamber by causing a shape memory material to contract andmove a piston; expelling liquid from the pumping chamber by causing theshape memory material to relax and move the piston; monitoring theposition of the piston; and controlling the position of the piston basedupon the monitored position.
 6. The method of claim 5 wherein monitoringthe position of the piston comprises sensing the condition of one ormore switches selectively activated depending upon the position of thepiston.
 7. The method of claim 5 wherein monitoring the position of thepiston comprises utilizing encoding indicia provided on the piston. 8.The method of claim 5 wherein monitoring the position of the pistoncomprises measuring the strength of a magnetic field.
 9. The method ofclaim 5 wherein monitoring the position of the piston comprisesmeasuring the capacitance between a stationary conductive element and aconductive element that moves with the piston. 10-14. (canceled)
 15. Apump comprising: a reservoir having a maximum volume; an actuatorcomprising a shape memory alloy that moves a plunger, the plungerdisplacing liquid from the reservoir; and a control system that utilizeslinear feedback to sense the position of the actuator and to manipulatethe actuator to provide doses of variable volume less than the maximumvolume.
 16. The pump of claim 15 wherein the control system comprises amicroprocessor.
 17. The pump of claim 15 wherein the control systemcomprises means for position detection.
 18. The pump of claim 15 whereinthe control system determines if the pump is primed or not.
 19. The pumpof claim 18 wherein if the pump is not primed the control system createsa warning. 20-27. (canceled)